The prior art methods for effecting mass transfer of a relatively small quantity of gas into a relatively large volume of liquid via bulk liquid agitation inevitably involves the irreversible and therefore inefficient conversion of mechanical energy to heat energy through viscous dissipation. The magnitude of the viscous dissipation loss is related to the viscosity of the liquid. If viscosity is low, then the viscous dissipation component is small. This condition is common to most wastewater treatment systems where total solids concentrations are low in the influent wastewater, e.g., 0.01 weight %, and the viscosity of the liquid-solid mixture is similar to that of water alone. However, as the solids concentration of the fluid increases, it is well recognized that the viscosity of the resultant mixture rises in an exponential fashion. As a result, in conventional mass transfer processes involving bulk liquid agitation at increasingly higher solids concentrations, the input shaft energy that is wasted through viscous dissipation rises accordingly and is schematically illustrated in FIG. 1. The problem becomes acute with a wastewater sludge hving at least 2.5 weight % solids, since such sludges not only have a high viscosity, but also exhibit pseudoplastic behavior.
Referring now to FIG. 1, in the prior art aeration systems input shaft power is used to induce the bulkwise interial convection of fluid 1, which substantially decays through the turbulent cascade process and produces mass transfer. In the case of a low viscosity fluid, a wastewater with a low total suspended solids (TSS), the energy lost through viscous dissipation is relatively small. The magnitude of viscous dissipation is represented by the vertical displacement on the FIG. 1 schematic. In the higher viscosity case (wastewater with a high TSS), however, the necessity for bulkwise inertial convection of fluid 2 to induce mass transfer results in a much greater loss of energy via viscous dissipation and a more inefficient aeration system. It would be advantageous, therefore, if the step of mass transfer via convective mixing could be avoided while maintaining the same degree of mass transfer. In this case, the shaft power would be directly transformed into the work necessary for mass transfer by what would correspond to a single stage energy cascade 3. FIG. 1 seems to indicate that there would be no viscous dissipation losses; obviously this represents the ideal case. However, if viscous losses can be made so small that the diagram substantially represents a real situation, the aeration requirement can be satisfied at relatively low power densities.
One object of the present invention is to provide an aeration method for pseudoplastic liquid-solid mixtures which promotes gas dissolution by essentially a single stage cascade.
Another object of this invention is to provide such an aeration method characterized by substantially lower power densities than heretofore achieved in such mixtures.
Still another object of this invention is to provide such an aeration method which can be practiced with inexpensive yet reliable equipment.
A still further object of this invention is to provide an aeration method useful in aerobic digestion of pseudoplastic wastewater sludge which promotes a high degree of gas dissolution at relatively low power densities.